On Wed, Sep 29, 2021 at 12:04:21AM +0000, Song Liu wrote: > Hi Peter, > > We have see the warning below while testing the new bpf_get_branch_snapshot > helper, on a QEMU vm (/usr/bin/qemu-system-x86_64 -enable-kvm -cpu host ...). > This issue doesn't happen on bare metal systems (no QEMU). > > We didn't cover this case, as LBR didn't really work in QEMU. But it seems to > work after I upgrade the host kernel to 5.12. > > At the moment, we don't have much idea on how to debug and fix the issue. Could > you please share your thoughts on this? Well, that's virt, afaik stuff not working is like a feature there or something, who knows. I've Cc'ed Like Xu who might have clue since he did the patches. Virt just ain't worth the pain if you ask me. > > Thanks in advance! > > Song > > > > > ============================== 8< ============================ > > [ 139.494159] unchecked MSR access error: WRMSR to 0x3f1 (tried to write 0x0000000000000000) at rIP: 0xffffffff81011a8b (intel_pmu_snapshot_branch_stack+0x3b/0xd0) > [ 139.495587] Call Trace: > [ 139.495845] bpf_get_branch_snapshot+0x17/0x40 > [ 139.496285] bpf_prog_35810402cd1d294c_test1+0x33/0xe6c > [ 139.496791] bpf_trampoline_10737534536_0+0x4c/0x1000 > [ 139.497274] bpf_testmod_loop_test+0x5/0x20 [bpf_testmod] > [ 139.497799] bpf_testmod_test_read+0x71/0x1f0 [bpf_testmod] > [ 139.498332] ? bpf_testmod_loop_test+0x20/0x20 [bpf_testmod] > [ 139.498878] ? sysfs_kf_bin_read+0xbe/0x110 > [ 139.499284] ? bpf_testmod_loop_test+0x20/0x20 [bpf_testmod] > [ 139.499829] kernfs_fop_read_iter+0x1ac/0x2c0 > [ 139.500245] ? kernfs_create_link+0x110/0x110 > [ 139.500667] new_sync_read+0x24b/0x360 > [ 139.501037] ? __x64_sys_llseek+0x1e0/0x1e0 > [ 139.501444] ? rcu_read_lock_held_common+0x1a/0x50 > [ 139.501942] ? rcu_read_lock_held_common+0x1a/0x50 > [ 139.502404] ? rcu_read_lock_sched_held+0x5f/0xd0 > [ 139.502865] ? rcu_read_lock_bh_held+0xb0/0xb0 > [ 139.503294] ? security_file_permission+0xe7/0x2c0 > [ 139.503758] vfs_read+0x1a4/0x2a0 > [ 139.504091] ksys_read+0xc0/0x160 > [ 139.504413] ? vfs_write+0x510/0x510 > [ 139.504756] ? ktime_get_coarse_real_ts64+0xe4/0xf0 > [ 139.505234] do_syscall_64+0x3a/0x80 > [ 139.505581] entry_SYSCALL_64_after_hwframe+0x44/0xae > [ 139.506066] RIP: 0033:0x7fb8a05728b2 > [ 139.506413] Code: 97 20 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b6 0f 1f 80 00 00 00 00 f3 0f 1e fa 8b 05 96 db 20 00 85 c0 75 12 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 56 c3 0f 1f 44 00 00 41 54 49 89 d4 55 48 89 > [ 139.508164] RSP: 002b:00007ffe66315a28 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 > [ 139.508870] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb8a05728b2 > [ 139.509545] RDX: 0000000000000064 RSI: 0000000000000000 RDI: 0000000000000010 > [ 139.510225] RBP: 00007ffe66315a60 R08: 0000000000000000 R09: 00007ffe66315907 > [ 139.510897] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000040c8b0 > [ 139.511570] R13: 00007ffe66315cc0 R14: 0000000000000000 R15: 0000000000000000
